Gamma knife



FIG. 1 is a front perspective view of a gamma knife showing our new ornamental design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom view thereof. 

CLAIM The ornamental design for a gamma knife, as shown. 